1. Field of the Invention
This invention relates to a magnetic recording system for recording information on magnetic recording media such as a magnetic disk, a magnetic tape and the like.
2. Description of the Prior Art
Magnetic recording technology has been widely utilized as a basic technique for recording a large amount of information such as external computer memory devices, VTR systems, DAT systems and the like.
In the conventional magnetic recording system, a longitudinal recording technique has been employed in which a remanent magnetization is formed in a direction parallel to the magnetic layer of a magnetic recording medium, so that magnetization-transition point is an information point.
However, in the longitudinal recording system, as the linear density (the recording density per unit length) increases, the remanent magnetization decreases due to a demagnetization effect. Because of this phenomenon, the recording density cannot be sufficiently increased.
To minimize the above-described disadvantages in the longitudinal recording system, a perpendicular recording system, in which the remanent magnetization is formed in a direction perpendicular to the magnetic layer plane so that magnetization-transition point is the information point, has been developed and applied to practical applications.
As for digital recording, the difference between the longitudinal recording system and the perpendicular recording system will be described in the following manner.
In the longitudinal recording system, as shown in FIG. 22(a), a repulsion force acts on the adjacent portions of the magnetization in a transition region. This is equivalent to a condition in which the demagnetization of the medium increases to a maximum at a magnetization transition point in the transition region. As a result, the magnetization decreases to a minimum at the magnetization transition point. In the longitudinal recording system, as the recording density becomes higher, the average demagnetization increases. Thus, in the longitudinal recording system, if the recording density is increased, a sufficient reproduced output can no longer be obtained. This inevitably restricts an increase of the recording density. In the perpendicular recording system, as shown in FIG. 22(b), the demagnetization decreases to a minimum in the transition region, and an attraction force acts on the adjacent portions of magnetization. As the recording density becomes higher the average demagnetization decreases. As a result, the enhancement of the recording density is basically not limited by the demagnetization. Therefore, the perpendicular recording system has been recognized as a recording system suitable essentially for high-density recording.
In general, when a conventional ring-type reproducing head is used, the reproduced waveform in the longitudinal recording system differs significantly from the reproduced waveform in the perpendicular recording system.
Further, when digital recording is performed in the ideal longitudinal recording system, a reproduced waveform P1 is obtained, as shown in FIG. 23(a). In FIG. 23(a), a single-peak pulse is generated with respect to the isolated transition of the magnetization. The peak of this pulse corresponds substantially to the magnetization-transition point A. Thus, the reproduction of information in the conventional longitudinal recording system has been performed in the following manner, the reproduced waveform is electrically differentiated, and then the zero-crossing point of the thus obtained differential waveform is detected.
Further, the actual waveform of the single-peak isolated reproduced waveform is not completely symmetrical and, accurately observed, its peak slightly deviates from the information-recorded point. This reduces the reproducing margin in the case of high-density recording.
In the case when digital recording is performed in the ideal perpendicular recording system, a reproduced waveform P2 is obtained, as shown in FIG. 23(b). A double-peak pulse is generated with respect to the magnetization-transition point A. Thus, theoretically, the reproduced waveform need not be differentiated for recovering the recorded information, i.e, only the detection of the zero-crossing point 0 of the reproduced waveform is needed.
In practice, even when the perpendicular magnetic recording is performed, not only the perpendicular magnetization component is formed in the medium, but the longitudinal magnetization component is formed in the medium. Thus, the reproduced waveform can become a distorted pulse P3, as shown in FIG. 23(c), i.e., the zero-crossing point 0 of the reproduced waveform inevitably deviates from the magnetization-transition point A substantially the same as the information point.
To solve the above-described disadvantage in the perpendicular magnetic recording system, various signal-processing systems have been disclosed; e.g., a system in which a Hilbert filter is used (reference: B. J. Langland and M. G. Larimore, "Processing of Signals from Media with Perpendicular Magnetic Anisotropy", IEEE Trans. on Magn. vol. MAG-16, No.15, 1980 and B. J. Langland, "Phase Equalization for Perpendicular Recording", IEEE Trans. on Magn. vol. MAG-18, No. 6, 1982), a double-differential system (reference: N. Aoyama, et al., "Bit Error Rate Characteristics for a Co--Cr--Ta Single Layer Perpendicular Recording Medium", J. of Magn. Soc. of Japan vol. 13, Supplement No. S1, 1989), a delayed signal-superimposing system (reference: T. Okuwaki, et al., "5.25-Inch Floppy Disk Drive Using Perpendicular Magnetic Recording", IEEE Trans. on Magn. Vol. MAG-21, No. 5, 1985) etc.
However, there is no system in which the essential remanent magnetization state per se has been improved.
Further, such signal-processing systems usually employ delay lines which are costly and cannot be easily miniaturized. Therefore, such systems are not suitable for practical applications.